Way a planet reduced energy (6)
I believe the answer is:
avenue
'way' is the definition.
(avenue is a kind of way)
'a planet reduced energy' is the wordplay.
'planet' becomes 'venus' (Venus is a planet).
'reduced' means to remove the last letter.
'energy' becomes 'e' (physics symbol as in E=mc2).
'venus' with its final letter taken away is 'venu'.
'a'+'venu'+'e'='AVENUE'
(Other definitions for avenue that I've seen before include "Broad tree-lined road" , "A broad wide street" , "Means of approach" , "A broad thoroughfare as in New York" , "Road lined with trees" .)